* Markdown path completions tests use mocked out fs
This updates the path completion tests to stop depending on the actual fs and instead use `IMdWorkspace`
* Update remaining tests
Renames in the markdown extension
This renames some types and splits up some files as part of an exploration towards a proper LSP. Changes:
- `SkinnyTextDocument` -> `ITextDocument`
- Moved `ITextDocument` to own file
- `MdWorkspaceContents` -> `IMdWorkspace`
This change reduces the number of times we retokenize a markdown file by doing the following:
- Use `MdTableOfContentsProvider` in more places
- Introduce a `IMarkdownParser` interface that lets us drop in a caching version of the tokenizer
